Parts and products can be tracked during manufacturing and marketing operations in order to facilitate commodity distribution, customizing product features, process control, stock control, sales control, quality control, and the like. Such tracking requires some means for marking or labeling parts and products so that such parts and products can be readily and accurately identified during tracking. For example, according to one approach, a label bearing identification information in human readable form (e.g., alphanumeric information) or machine readable form (e.g., bar code information), or both, is attached to parts running on a production line. The label is read at one or more process stations so that specific operation steps can be carried out according to work schedules corresponding to the identification information.
In some industries, printed paper labels are used for tracking purposes. However, for other industries, (e.g., steel making industries and cathode ray tube industries) production lines operate at temperatures well above ambient conditions (e.g., above 250.degree. C., or even above 400.degree. C.). For example, the manufacture of a glass cathode ray tube typically involves temperatures in the range from about 400.degree. C. to about 1000.degree. C. Paper labels do not survive at these temperatures. Consequently, labels that can be easily attached to parts and maintain their integrity at such temperatures are needed.
Many parts have nonplanar surfaces to which a label is attached in order for such parts to be tracked. For example, a cathode ray tube includes a glass cone component and a glass faceplate component, both of which have curved, nonplanar surfaces. Consequently, not only is there a need for temperature resistant labels, but there is also a need for temperature resistant labels which are conformable to nonplanar substrates at application temperatures.
Another concern relates to the durability of labels. It is most desirable if a label is durable enough to survive the entire production process without requiring interim replacement. Unfortunately, many of the previously known labels adapted for high temperatures become disfigured and/or unreadable too easily and must be replaced several times during production in order to preserve tracking capabilities. Other labels, although durable, have a tendency to delaminate and fall off of their substrates. Consequently, labels with both excellent durability and tenacious bonding characteristics would be desirable.
As a practical matter, production yields seldom reach or are maintained at 100%. For example, parts can break, fail to meet specifications, or suffer from other defects that necessitate discarding or recycling of the parts. Recycling is often more desirable than discarding bad parts, particularly when the raw materials incorporated into the parts are relatively expensive. However, before recycled materials can be returned to the production line, contaminants are preferably removed to maintain the quality of parts formed from recycled material. Unfortunately, many of the previously known labels would contaminate the recycled supply and must be removed. Labels comprising heavy metal atoms such as Pb, Hg, As, Co and Cr(VI), whether present in pure form or in the form of oxides, are of particular concern due to the toxicity associated with such materials. Oxides of heavy metals such as Pb, Cd, Hg, and/or As are known to be incorporated into glassy phases as one approach for achieving low softening points (e.g., softening points below about 350.degree. C.). Oxides of heavy metals such as Co and Cr are known to be used for providing black color. Consequently, articles having a label comprising one or more these heavy metals are typically recycled by delabeling and separating the less desirable heavy metals containing labels from the articles. These steps can be costly and/or extremely difficult, particularly if the bond between the article and the label is especially tenacious. In addition, heavy metals may be dispersed in air when the labels containing such metals are heated to high temperatures. Accordingly, it would be desirable to provide a label substantially free of these heavy metal atoms.